Regulatory CD4 T cells minimally migrate to inflamed allografts in the absence of effector T cell responses.

CD4+Foxp3+ regulatory T cells (Treg) are essential for maintaining immune tolerance in both lymphoid and non-lymphoid tissues, including transplanted organs. Understanding the mechanisms that regulate Treg suppressor function and tissue migration remains a major focus in transplantation immunobiology. Here, we show that under minimal effector T cell (Teff) response settings, Treg are unexpectedly inefficient at migrating to inflamed allografts despite circulatory presence. In contrast, Teff infiltration into inflamed allografts induces a multifold increase in intragraft Treg accumulation. These observations were confirmed in two lympho-replete murine models in which Treg trafficking can be assessed with or without concurrent Teff activity. Intravital microscopy reveals that while Tregs transferred alone remain confined to perivascular spaces, the presence of Teff enables Treg migration deep into graft tissue. Mechanistically, Teff increase graft inflammation and Teff-driven Treg migration to allografts requires chemokine receptor signaling in Treg. In addition, we demonstrate that Teff-conditioned media directly promote chemokine receptor dependent Treg transmigration in vitro. Collectively, our findings demonstrate that Treg exhibit restricted migration and accumulation within inflamed allografts under controlled Teff responses, emphasizing the need to optimize Treg therapy designs to improve their trafficking to allografts under immunosuppression.